Aromatic carboxylic acids such as benzene dicarboxylic acids, naphthalene dicarboxylic acids, and others are commercially valuable as the raw materials for manufacture of polyesters which are used to manufacture fibers, films, resins, and many other useful articles. Aromatic carboxylic acids can be produced via liquid phase oxidation of aromatic hydrocarbon feedstock. U.S. Pat. No. 2,833,816, incorporated by reference herein, discloses liquid phase oxidation of xylene isomers into corresponding benzene dicarboxylic acids in the presence of bromine using a catalyst having cobalt and manganese components. As described in U.S. Pat. No. 5,103,933, liquid phase oxidation of dimethylnaphthalenes to naphthalene dicarboxylic acids can also be accomplished in the presence of bromine and a catalyst having cobalt and manganese components.
As used herein, “aromatic hydrocarbon” preferably means a molecule composed predominantly of carbon atoms and hydrogen atoms, and having one or more aromatic ring, particularly benzene, toluene, dimethyl benzenes, naphthalenes, methyl naphthalenes and dimethyl naphthalenes. Aromatic hydrocarbons suitable for liquid-phase oxidation to produce aromatic carboxylic acid generally comprise an aromatic hydrocarbon having one or more substituent groups, at least one of which is oxidizable to a carboxylic acid group. As used herein, “aromatic carboxylic acid” means an aromatic hydrocarbon having one or more substituent groups, at least one of which is a carboxyl group.
In a typical liquid phase oxidation process, an aromatic hydrocarbon feedstock and a solvent are reacted with an oxidant gas in the presence of a bromine promoter and catalyst. Typically, the solvent comprises a C1-C8 monocarboxylic acid, for example acetic acid, benzoic acid, or mixtures thereof with water. Typically, air is used as the oxidant gas. The particular aromatic hydrocarbon used depends upon the desired aromatic carboxylic acid. For example, in the production of benzene dicarboxylic acids, the corresponding xylene isomer is used as aromatic hydrocarbon feedstock. Ortho-xylene is oxidized to produce phthalic acid, meta-xylene is oxidized to produce isophthalic acid, and para-xylene is oxidized to produce terephthalic acid.
Aromatic carboxylic acid product obtained from a liquid phase oxidation process may be subjected to a subsequent purification process. The purification process may include treating the aromatic carboxylic acid product with hydrogen gas in the presence of a hydrogenation catalyst.
The presence of isomers or other species in an Aromatic hydrocarbon feedstock can impact a particularly desired oxidation process. Costly separation procedures are often employed to reduce the presence of such isomers or other species in an aromatic hydrocarbon feedstock for commercial oxidation processes. For example, meta-xylene may be separated from para-xylene to form a para-xylene feed of sufficient purity for use in commercial liquid phase oxidation processes for the production of terephthalic acid. Such separation procedures can be difficult and costly and, consequently, the supply of such aromatic hydrocarbon feedstocks can be costly. It would be advantageous to employ alternative processes for producing aromatic carboxylic acids from more easily obtainable aromatic hydrocarbon feedstocks.
U.S. Pat. No. 1,866,717 to Meyer, et al., incorporated by reference herein, discloses a method of producing aromatic carboxylic acids by allowing CO2 to react with an aromatic hydrocarbon in the presence of aluminum chloride. Yields from the reactions described by Meyer, et al., (based on AlCl3) are very low, ranging from about 5% at atmospheric pressure to about 15% or 20% at 200 atmospheres (20.27 Mpa).
U.S. Pat. No. 3,138,626 to Calfee, et al., incorporated by reference herein, discloses the carboxylation of aromatic hydrocarbons with CO2, aluminum chloride, and finely divided aluminum or zinc to produce aromatic carboxylic acids. Addition of zinc or aluminum metal in finely divided form increases the yield of carboxylated product formed during aluminum chloride catalyzed carboxylation of aromatic hydrocarbons by CO2 compared to the yield in accordance with Meyer, et al. Addition of finely divided aluminum metal results in yields of carboxylated product (based on AlCl3) of about 23% at atmospheric pressure and from about 55% to about 60% at about 200 atmospheres of pressure (20.27 Mpa). Unfortunately, finely divided aluminum or zinc is costly. It would be advantageous to achieve an increased yield of carboxylated product without adding finely divided aluminum or zinc.
In processes for the carboxylation of aromatic hydrocarbons, a reaction mixture of aromatic hydrocarbon feedstock and a Lewis acid are introduced into a reactor and the reactor is then pressurized with CO2. Zinc or aluminum powder, if used, would be added to the reaction mixture before pressurizing the reactor with CO2. We have found that, surprisingly, premixing CO2 and the Lewis acid significantly improves yield of carboxylated product without the addition of zinc or aluminum powder.